1!) THOMAS SAY FOUNDATION 



wings. Three at least out of four will usually be found 

 in agreement. 



Much confusion in key-making in the past has arisen 

 out of too great dependence on such characters as the 

 hing of the radial sector and the number of cross- 

 veins in the several series. Occasionally the normal male 

 of a species would go by the key into one genus and the 

 female into another. In order to determine what char- 

 acters may be trusted and how far trusted, the senior 

 author has made variation studies of a few series of 

 wings in the Perlidae, and the results are presented in 

 the following pages. 



Variation in Pteronarcys dorsata. — This was studied in 

 20 males and 8 females. The branching of the radial sec- 

 tor was as follows : 



3 4 5 6 7 



Times occurring $ 3 28 42 7 



occurring 9 12 13 6 1 



Both $ and 9 3 40 55 13 1 



From this it appears that the radial sector is typically 

 five branched: on the five typical branches extra twigs 

 appeared in 19 out of the 112 wings. 



The branching of cubital vein (vein M being inter- 

 preted as two branched) was as follows: 



No. of branches., 12 3 4 



Times occurring <$ 1 31 41 4 



Times occurring 9 1 8 20 3 



Both o and 9 2 39 61 7 



Extra twigs occurred on the first, second and third 

 longest of these branches 52, 13 and 2 times respectively. 



Longitudinal rows of cells within the cubital fork oc- 

 curred as follows : 



No. of cell rows 1 V 2 2 2y 2 3 



Times occurring S and 9 - i 34 19 2 



with no significant differences between £ and $ . 



The number of crossveins in cell M varied about a dif- 

 ferent mode in fore and hind wing, as shown in the fol- 

 lowing figures: 



No. of crossveins I 2 3 4 5 6 7 8 9 10 11 



.Times occurring, f. wing 1 13 1!' 15 2 5 1 



Times occurring, h. wing....] 8 25 16 6 



The number of cells in the irregular meshwork in the 

 space between veins R and R2 beyond the cord varied 

 from 3 to 12, with the mode at 5 but with chaotic dis- 

 tribution of the variants. 



